Laser technology is now widely used to more accurately and efficiently perform functions previously performed using other means. One particular field in which the laser has been especially useful is for cutting, drilling, and/or shaping materials. Specifically, lasers are used in the manufacturing and preparation of circuit boards, semiconductor wafers, and other articles used in the computer industry. Often, lasers are used because they perform functions faster, more accurately, with higher quality and to conform to stricter tolerances than mechanical processes are capable of achieving.
Such holes or passages are often formed in parts used in the computer industry to, among other purposes, selectively remove portions of conductive layers, to form passages for the attachment of other components, and/or to connect various layers of a substrate. Although a variety of types of lasers may be used for this application, it has been found that an excimer laser often produces favorable results when employed for the above described purposes. Excimer laser drilling, especially as related to high density circuit packaging technology, has some very significant positive attributes, such as the ability to form extremely small holes, applicability to numerous material sets, and the production of clean bottomed holes.
On the other hand, excimer lasers in particular have a number of drawbacks. Specifically, excimer lasers have a reputation for low through put simply because many pulses generally are required to form a hole. Additionally, excimer lasers include a relatively low pulse repetition rate.
To accomplish the formation of multiple holes in a substrate, requires the movement of the laser beam about the substrate either by moving the substrate or controlling the beam in some manner. A system in which the drilling is done while the part is actually moving is known as an "on-the-fly" system. With an on-the-fly system, a laser beam may be pulsed on and off as the laser source and substrate move relative to each other. The beam moves over the entire surface and then repeats this movement until the holes are all completely drilled. Using one beam, it is relatively simple to drill holes only in certain spots and not others, but the processing of a large substrate can take a long time.
To increase the speed with which a substrate may be processed, a plurality of beams may be caused to act on the substrate. For instance, one large beam may be divided into a plurality of smaller beams, such as a row of five beams. This row of beams may move across a substrate such that the first beam in the row may be caused to hit the first of a row of hole sites on a substrate. The row of beams is then caused to move so that the first beam strikes the second hole site and the second beam hits the first hole site. The row of beams moves down the substrate so that the beams sequentially hit each hole site on the substrate.
Such multi-beam on-the-fly drilling greatly increases the processing speed. However, unless every substrate processed has the same pattern of holes and the holes are all lined up and there are not places in which holes are not desired, some method must be used to selectively turn the beam on and off. The need for selectively turning the beam on and off increases as the number of beams the original laser beam is split into increases. Since only some of the split beams need to be blocked, the beam cannot be controlled electronically since all of the beams are derived from a single source. Therefore, mechanical means must be used to selectively block the laser beam. It has been contemplated to use a shutter or other similar means to mechanically block the laser beam from hitting the substrate. However, such systems are mechanically complex and can be expensive and difficult to operate at the high pulse rate typical in these laser processing applications.
In an attempt to solve the above described problems inherent with laser processing of substrates, a combination of hardware and software has been employed using nearly every pulse produced by the laser. However, the formation of holes in many materials requires numerous pulses from the laser, frequently forty or more. The requirement of numerous pulses plus the limitation of the finite pulse rate inherent of the lasers results in a relatively long process time even when using the on-the fly system.
As described above, in an attempt to reduce the process time and increase the through put of the on-the-fly drilling system, the use of multi-beam drilling has been proposed. Such multi-beam drilling is made possible with a relatively high power excimer laser due to the inherent large beam size produced by the laser. However, a large fraction of the available laser energy may be discarded through the use of a beam shaping aperture located in the beam path.
Additional problems arising using a multi-beam system include the introduction of the complication that one or more of the multiple beams must be turned off, redirected, or otherwise blocked to achieve the needed goal of forming holes in certain locations but not in others. In an attempt to solve this problem, a combination hardware and software controlled blocking has been proposed including a plurality of actuators selectively blocking off one or more of the beams, thereby allowing holes to be selectively drilled at various locations on the substrate but not others. However, not only is this combination system mechanically complex but it also involves significant hardware and software resources to operate. The blockers used must be low mass in order to move quickly and also must have a sufficient thermal stability to survive repeated exposure to the laser pulses.